Geology

Burra, along with a fair bit of Shetland and Scotland, is made up (in part) of a group of metamorphic rocks known collectively by geologists as the Dalradian Supergroup, or just ‘Dalradian’ rocks. The fact that most of Burra’s rocks are metamorphic means that they started life as something else and were altered or re-crystallised to their present condition by heat and pressure over many millions of years. Today, after many hundreds of millions of years of rock formation followed by erosion, we see quite a variety of rock types exposed on Burra and the neighbouring islands.

If we go back some 750 million years to a time when most of the Earth’s landmasses lay south of the Equator, we would find that our metamorphic rocks were originally sediments in the form of sands and lime (calcium carbonate) rich muds deposited horizontally on a tidal shelf. This tidal shelf lay along the shore of a shallow sea that had developed as a large continental mass was split in half by tectonic forces.

The shallow sea eventually opened and deepened to become an ocean (the Iapetus Ocean) which closed again some 200-100 million years later. These plate tectonic movements buried sands so they became metamorphosed by heat and pressure to form psammites and the lime muds to become crystalline limestone (calcite marble). Today we see the psammites as the grey rocks of the west coast of West Burra and Trondra, often folded and cut by white quartz veins. The limestone breaks down to give a much more fertile soil than psammite or gneiss and this is very noticeable where crofts have been established such as at East Hogaland or on the west shore of Lang Sound.

Following the ocean closure the psammites lay deep beneath the Earth’s surface within a mountain chain (the Caledonian Mountains). The once flat and horizontal beds of rock had became squeezed, contorted and tilted into almost a vertical position by the mountain building pressures. Some of these rocks melted due to the presence of hot hydrothermal fluids circulating upward through them from great depths within the Earth’s crust. The resulting magma worked its way upward through the psammites to accumulate and cool again to form large pods of pink granite or pegmatite veins. Pink pegmatite veins of various thickness can be seen across much of Burra but the best exposure of the pink pods of granite are in the cliffs of Kettla Ness from Groot Ness to The Heugg.

Psammites in close proximity to the rising magma were further metamorphosed by the added heat and hot fluids so in places all traces of the original sediments were lost as they re-crystallised to become coarse-grained gneisses. This gneiss makes up much of East Burra.

About 400 million years ago the great mountain chain was at its height and the rocks we see today as Burra were buried many kilometres deep in the mountain’s roots. About then molten rock (magmas) from even deeper down found its way up into these older metamorphic rocks. Over many millions of years these intrusive magmas slowly cooled and solidified deep in the mountains. Today we see them exposed at the surface as various types of ‘granitic’ rock such as Monzonite, and Adamellite.

This cooling process took place over a very long time so the rock forming mineral content of the granites changed (evolved) and this can be detected on a traverse across the intrusion from its earlier to its later stages. Monzonite is a less evolved and earlier rock than Adamellite. A change in crystal size is also seen in the rocks of such an intrusion and can act as a pointer to its cooling history. Crystals have less time to grow where heat loss is greatest, i.e. on the outside nearer the cold surrounding rock. You can see good examples of Adamellite with large crystals of potassium feldspar in the rocks of Fugla Ness.

The ‘granitic’ rocks of Papa, Oxna and Hildasay as well as Atla Ness and Fugla Ness are part of a much larger intrusion which lies mostly under the sea but extends from Lady’s Holm off Scatness through Spiggie to Hamnavoe and the offshore islands and north to Bixter and Aith Voe.

Over the last 350 million years the rocks we now see on Shetland and Burra have taken a tectonic journey from the Equator to 60N. Some evidence of this tectonic journey can be seen in the great geological faults that run almost north-south through Shetland, one of which passes through Tingwall and down Cliff Sound. Cliff Sound is the drowned part of the same valley as Tingwall. This great valley formed because limestone is much more prone to erosion by water than the gneisses of Burra or the phyllite of the Clift Hills. Perhaps a great river flowed through this valley sometime in the geological past before it was modified by ice flow during later glaciations.

In that time sea levels have risen and fallen and ice ages have come and gone so that erosion and sea level rise has shaped the Burra and islands we see today. At the end of the last glaciation, some 10,000 years ago, Burra and the islands were attached to Mainland across low lying valleys as sea level was about 120m lower than it is today. However sea level rose rapidly so by 5000 years ago when the first inhabitants reached Shetland sea level was about 10m lower than today. Even over the next 2000 years as Burra became settled for the first time, both islands were probably joined to Trondra and to Mainland but eventually rising sea levels claimed the last of the low lying land to form sounds and create the pattern of islands we know today.

More on Shetland geology can be found at:

Shetland Landscapes: www.fettes.com/shetland

Geopark Shetland: http://www.geoparkshetland.org.uk

Shetland Geotours: www.shetlandgeology.com/